By Dr. Aude Watrelot –
As explained in the previous newsletter (April 2020), “tannins in grapes”, the differences between white and red wines are due to polyphenols, as they play a major role in the wine color, texture, and taste. Among the large group of polyphenols, anthocyanins are red pigments responsible for the red-purple color of grape skins, and sometimes grape flesh.
In this research winemaking focus, anthocyanins will be explained from the chemical structure to their role and importance in red wine.
What are anthocyanins?
- Chemical structure:
Anthocyanins are a type of polyphenol from the flavonoid group that is the red pigment found in grape skins and sometimes in the flesh. “Teinturier” is the name of the red grape that contains anthocyanins in the flesh leading to red-purple berries. As part of the flavonoid group, their chemical backbone structure is composed of two benzene cycles (A and B) on the left and right side of the central oxygenated heterocycle C. On the oxygen there is a positive charge called the flavylium cation when the pH of a solution is below 3 (Figure 1). As it has been discussed in the newsletter of tannins in grapes, the cleavage of a condensed tannin molecule leads to a flavanol such as catechin, epicatechin, epigallocatechin and epicatechin gallate. When the unit is an extension unit and after several chemical reactions, it can lead to an anthocyanidin (such as cyanidin or delphinidin) under acidic and hot conditions. An anthocyanidin does not have a molecule of sugar attached to the backbone structure, whereas an anthocyanin does. In grapes, anthocyanins can have one or two sugar groups linked at the position 3 and 5. The number of hydroxyl groups (-OH) and methoxy groups (-OCH3) of the B ring also characterizes anthocyanins (Figure 1). The most prevalent anthocyanin in Vitis vinifera grapes is the malvidin-3-O-glucoside meaning that a glucose is glycosylated at the position 3 and that the B ring has 1 hydroxyl group and 2 methoxy groups. Depending on the chemical structure, the color varies. As an example, the delphinidin-3-O-glucoside tend to be blue-ish compared to the cyanidin-3-O-glucoside that is orange/red-ish (Figure 1).
Figure 1. Chemical structure of flavylium cation form of anthocyanins including malvidin-3-O-glucoside, malvidin-3.5.diglucoside, pelargonidin-3-O-glucoside and delphinidin-3-O-glucoside.
- Biosynthesis in grapes :
Grape anthocyanins are biosynthesized during the berry ripening after véraison until harvest maturity. Anthocyanins can also be found in large quantities in the leaves at the end of the growing season. At véraison, the red berries start to turn pink and then red due to the biosynthesis of anthocyanins in the skin. Similar to tannins, many environmental factors such as sun exposure, temperature, ripening stage, vintage and the grape variety have an impact on the amount and composition of anthocyanins in grapes. In Vitis vinifera grapes, mono-glucosides (1 sugar on the molecule) are primarily present. In comparison, interspecific hybrid grapes contain mono-glucosides and di-glucosides anthocyanins. As an example, Cabernet sauvignon wine contains about 1,500 mg/L of anthocyanins compared to Pinot noir wine that contains about 100 mg/L. In Frontenac grape juice, the concentration of anthocyanins varied from 270 to 6,000 mg/L. This concentration tends to be higher in grapes that were sun exposed after leaf removal (Burtch and Mansfield, 2016; Scharfetter et al., 2019).
Why are anthocyanins important in wine?
As explained above, anthocyanins are responsible for the red-purple color of grapes and red wine color. Depending on the chemical structure and medium conditions, anthocyanins are more or less stable. Because the color of a red wine positively relates to the red wine grade, it is important to be aware of the color stabilization processes.
- pH is the first factor that can lead to different chemical structure and therefore different color. At a very low pH, meaning high acidity, the flavylium cation form is major and the color tends to be deep red. When the pH increases to wine pH 3.6-3.8, the structure of the molecule changes as well as the color. At a neutral pH (about 7) and basic pH the color tends to be black to blue, also due to the chemical structure modifications.
- Another medium factor is the presence of sulfur dioxide (SO2). Molecular SO2 is used to protect wine against microbial spoilage and oxidation. The bisulfite form (HSO3-) can bind to free anthocyanins and form anthocyanin-bisulfite adduct. This tends to bleach the color of free anthocyanins.
- A third factor in the stabilization of color in wine is the presence of other compounds such as other anthocyanins or flavanols or tannins, with or without acetaldehyde acting as co-pigments. Copigmentation is the interaction between molecules to form stable complexes. During wine aging the formation of new pigments as seen in Figure 2 leads to more stable molecules that would be less bleachable by sulfur dioxide. They are also less prone to degradation, therefore leading to a more stable color.
Figure 2. Formation of catechin-malvidin-3-Oglucoside co-pigment, more stable during time.
What happens during winemaking?
Anthocyanins are more soluble in an aqueous solution than in an ethanolic solution. Therefore, in order to enhance the extraction of free anthocyanins from grape skins, crushed grapes can be macerated at 39 to 50 F (4 to 10 C) for a few hours to 10 days, which is called a cold soak. As explained above, free anthocyanins are not stable and can react with HSO3-. Also depending on the pH of the medium, the color can be more orange-ish than brick or red-purple.
During alcoholic fermentation, alcohol is produced and the extraction of anthocyanins from grapes is reduced due to their lower solubility in alcohol. At the same time, condensed tannins are extracted, and co-pigments are formed through oxidation reactions. Acetaldehyde, formed by the oxidation of ethanol by yeasts and some bacteria, tends to accelerate the formation of co-pigments or co-pigmented anthocyanins (Figure 3).
Figure 3. Extraction of anthocyanins and formation of co-pigmented anthocyanins during red winemaking steps.
It has also been shown that yeast mannoproteins are able to interact with anthocyanins and stabilize wine color. Moreover, the use of oak chips during maceration could help in the modulation of red wine color stabilization, due to the hydrolysable tannins released from wood.
Further work is needed on these two aspects, especially on interspecific hybrid grapes that are rich in mono- and di-glucosides anthocyanins.
If you have any suggestions or questions about the research/winemaking topic, feel free to contact me at email@example.com.
References to learn more:
Burtch, C., & Mansfield, A. K. (2016). Comparing Red Wine Color in V. vinifera and Hybrid Cultivars. 6.
Burtch, C. E., Mansfield, A. K., & Manns, D. C. (2017). Reaction Kinetics of Monomeric Anthocyanin Conversion to Polymeric Pigments and Their Significance to Color in Interspecific Hybrid Wines. Journal of Agricultural and Food Chemistry, 65(31), 6379–6386. https://doi.org/10.1021/acs.jafc.6b05331
Lorrain, B., Chira, K., & Teissedre, P.-L. (2011). Phenolic composition of Merlot and Cabernet-Sauvignon grapes from Bordeaux vineyard for the 2009-vintage: Comparison to 2006, 2007 and 2008 vintages. Food Chemistry, 126(4), 1991–1999. https://doi.org/10.1016/j.foodchem.2010.12.062
Morata, A. (Ed.). (2018). Red Wine Technology (1 edition). Academic Press.
Scharfetter, J., Workmaster, B. A., & Atucha, A. (2019). Preveraison Leaf Removal Changes Fruit Zone Microclimate and Phenolics in Cold Climate Interspecific Hybrid Grapes Grown under Cool Climate Conditions. American Journal of Enology and Viticulture, 70(3), 297–307. https://doi.org/10.5344/ajev.2019.18052